Generally widely used optical information reading apparatuses such as a bar code reader and the like include an apparatus using a light emitting diode (LED) as a light source, using a CCD image sensor as a photodetector element to perform photoelectrical conversion, and processing a resulting converted electric signal, an apparatus using laser light as a light source, receiving and photoelectrically converting reflected light of the laser light, and processing a resulting electric signal, and so on.
These optical information reading apparatuses are typically configured as shown in FIG. 43. Specifically, an optical information reading apparatus 100 comprises a light emitting block 101 having a light source such as an LED, a semiconductor laser, or the like, a light receiving block 102 composed of a CCD image sensor or the like, and a signal processing block 103.
The light emitting block 101 projects light to an object to be read 200 having portions with different light reflectances such as a bar code symbol 201 or the like, the light receiving block 102 receives reflected light from the object to be read 200 and photoelectrically converts it, the signal processing block 103 processes a resulting electric signal, recognizes information of the bar code or the like, and sends its code data to a host apparatus 300 such as a personal computer or the like.
The signal processing block 103 comprises an amplification circuit 104, a binarization circuit 105, a decoder 106, and an interface 107. Conventionally, a signal has generally been subjected to analog processing in the amplification circuit 104 and the binarization circuit 105, and to digital processing in the decoder 106 and the interface 107.
The decoder 106 recognizes a bar code or the like, converts it into code data, and sends the data to the host apparatus 300 via the interface 107.
Such an optical information reading apparatus 100 using an LED as the light source of the light emitting block 102 and using a CCD image sensor as the light receiving block 102 is found, for example, in JP H6-187480 A.
Further, an optical information reading apparatus similarly using a CCD image sensor as a light receiving block, in which a calculation circuit block being a signal processing block for processing an electric signal which has been photoelectrically converted in the light receiving block includes a logarithmic amplifier, a slice signal generation circuit, and a comparison circuit so as to enable control capable of quickly responding to a transient or instantaneous change of disturbance light while maintaining wide dynamic range characteristics, thereby reading accurate optical information, is found in JP H6-195496 A.
Besides, an optical information reading apparatus using a laser light source such as a semiconductor laser as a light source of a light emitting block is found, for example, in JP H11-259592 A. Also in the optical information reading apparatus, signal processing after photoelectric conversion in a light receiving block is analog processing through use of an amplification circuit and a binarization circuit and thereafter digital processing in a decoder and an interface.
The signal processing in the above-described conventional optical information reading apparatuses, however, have the following problems:                {circle around (1)} the processing performance tends to vary with a change in amplification rate, temperature, or the like;        {circle around (2)} when a signal to be binarized in the binarization circuit is subjected to analog processing, the processing performance tends to vary due to variations of parts of the binarization circuit; and        {circle around (3)} the conventional optical information reading apparatus including the processing circuit of this kind has a large number of parts and therefore tends to cause variations in accuracy of quality, have a relatively large size, and increase in cost.        
The present invention is made to solve those problems, and an object of the invention is to improve the signal processing performance in the above-described optical information reading apparatus, so as to increase the reliability of reading information such as a bar code symbol or the like being an object to be read and, in addition, to realize a reduction in size and cost of the reading apparatus.
As more specific points, the invention has the following objects:                {circle around (1)} to eliminate variations in processing performance by avoiding influence of variable factors such as a change in amplification rate, temperature, and so on;        {circle around (2)} to eliminate variations in processing performance by reducing the number of parts of a processing circuit;        {circle around (3)} to improve reading performance by avoiding influence of the printing quality (gradation or the like) of a bar code symbol or the like being an object to be read;        {circle around (4)} to reduce size and manufacturing cost of the optical information reading apparatus itself;        {circle around (5)} to enable easy selection of various kinds of digital processing by employing a new calculation circuit (multipurpose processing circuit); and        {circle around (6)} to reduce voltage and power consumption of the calculation circuit block.        